The present invention relates to the field of creating glossy surfaces on sheet materials, and more particularly to a device for controlling the cross-directional gloss profile of a paper sheet
One of the parameters used in grading sheet materials is the gloss of the surface. For example, in the paper production process, various grades of paper having different surface gloss are produced to suit various applications. Typically, bulk paper is produced in a continuous sheet and wound in rolls having dimensions on the order of 25 feet or more in the cross direction (i.e. across the width of the sheet). Uniformity of gloss on the paper surface is often desirable or necessary. For example, in the situation where the roll of paper is cut to size for making various paper products, the consistency of the gloss of the individual paper items is dependent upon the uniformity of the gloss of the original bulk paper roll.
Paper production typically involves a calendering process which includes pressing paper material between calender rolls to obtain the desired physical characteristics. For example, calendering paper can change its density, thickness and surface features, including gloss. Gloss is typically created on the surface of the paper by applying steam to the paper surface, followed by pressing the paper between a series of calender rolls, typically arranged in a stack of alternating hard polished steel rolls and soft or reslient rolls made of cotton. The paper absorbs the steam and paper fibers at the surface are softened by the heat and moisture. As the polished steel calender roll comes into contact with the paper surface that has been treated with steam, it smooths the treated paper surface by pressing and rubbing actions against an adjacent cooperating roll to produce a glossy finish on the side of the paper facing the steel roll. The degree of gloss is dependent on the amount of moisture and heat and hence the amount of steam applied to the surface.
A common problem encountered in making a glossy finish using a steam treatment is the non-uniformity of the gloss finish of the calendered material. Localized variations in the amount of steam applied to the surface of the bulk paper may affect the uniformity of the gloss finish. Also, there are other variables in the calendering process such as temperature and calender roll pressure that may affect the amount of steam required for a particular degree of gloss. A more uniform gloss finish could be obtained if the amount of steam directed at the material could be controlled to vary independently for different sections of the paper surface.
Another common problem associated with the application of steam in calendering a gloss finish is that excess steam that has not been absorbed by the paper condenses on cool surfaces of the adjacent structure of the calender system. For example, the steam may condense on the steel calender roll which will wet the paper a the steel roll contacts the paper. The extra moisture from the steel calender roll in addition to the moisture applied directly to the sheet from the steam supply will affect the moisture distribution and hence the gloss finish and other physical properties of the paper. In addition, excess steam may condense on a cool portion of the paper surface at a location where steam treatment is not intended, thereby affecting the gloss profile. Moreover, steam which condenses on cool surfaces forms water droplets which may drip on the paper as it passes through the system of calender rolls, thereby affecting the desired properties of the paper.
Certain types of devices for gloss finishing paper sheets include a steam box located within the pocket formed by paper sheet traveling from a polished steel roll in the calender stack to an adjacent idler roll, and from the idler roll back to the steel roll. The steam box is necessarily made small in order to fit within the pocket. The steam box directs steam at the paper surface in zones in the cross direction of the paper. Steam is supplied to the individual zones from an external steam generator. Pneumatically actuated control valves are used to control the amount of steam supplied to each zone.
The previously described device, however, is subject to certain limitations and inefficiencies. For example, due to the small size of the steam box, the steam control valves are located remotely from the steam box, i.e. outside of the steam box. Thus, the controlled flow of steam is delivered to each zone of the steam box through individual tubes leading from the control valves. However, as the number of zones increases, the number of tubes delivering steam to the various zones must also increase and thus the tube diameters decrease. These small diameter tubes may cause an uneven flow of steam to the steam box. For example, due to the small cross sectional area of the tubes, and the long length of the tubes leading from the remote steam control valves to the steam box, the heat energy of the steam may be lost through the walls of the tubes. As a result, condensation may occur within the tubes, thereby causing an undesirable two phase flow, i.e. a mixture of steam and water. Such flow is difficult to control and thus the actual amount of steam delivered to each zone is unpredictable.
In addition, the resistance to flow in the tubes increases with a decrease in tube diameter Hence, excessive power is required to deliver steam through the smaller tubes running between the steam box and the remote valves. Furthermore, due to the long length of tubes running between the steam box and the remote valves, there is a long lag in the response time when adjusting the amount of steam applied to the paper by means of the distant control valves. Consequently, with these types of devices, it is difficult to control the changes in steam distribution on the paper surface at a precise moment.